Processes for preparing (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation of at least one saturated hydrocarbon precursor compound, by charging a catalyst bed which is disposed in a reactor and whose catalytically active composition is a multimetal oxide which contains the elements Mo and V, at least one of the elements Te and Sb, and at least one of the elements from the group consisting of Nb, Ta, W and Ti, and whose x-ray diffractogram has reflections h, i and k whose peak locations are at the reflection angles (2Θ) 22.2±0.5° (h), 27.3±0.5° (i) and 28.2±0.5° (k) at elevated temperature with a charging gas mixture which, in addition to the at least one saturated hydrocarbon precursor compound, molecular oxygen as an oxidant and steam as a promoter, also comprises at least one diluent gas which is substantially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation, are known (cf., for example, EP-A 1192987, DE-A 10122027, JP-A 2000-256257, EP-A 608838, EP-A 1193240, EP-A 1238960, EP-A 962253, JP-A 10-36311 and EP-A 1254706).
According to the teaching of these prior art documents, it is particularly advantageous that steam is present in the charging gas mixture, since, inter alia, it promotes the selectivity of (meth)acrylic acid formation (cf., for example, EP-B 608838 page 4), and according to the teaching of the prior art, an increasing amount of steam in the charging gas mixture has an increasingly advantageous effect. The activity of the catalytically active multimetal oxide composition is also favorably influenced by the presence of steam.
It is also known of the catalytically active multimetal oxide compositions used that the catalyzed partial oxidation is brought about by the oxygen present in the multimetal oxide active compositions, and has to be constantly resupplied to the multimetal oxide active composition by way of a redox reaction by means of the molecular oxygen present in the charging gas mixture.
Increased molar ratios of molecular oxygen to saturated hydrocarbon precursor compound in the charging gas mixture are therefore generally regarded as advantageous.
However, a disadvantage of the aforementioned procedures which are recommended as particularly advantageous is that the (meth)acrylic acid does not occur as a pure substance in the product gas of the heterogeneously catalyzed partial gas phase oxidation but rather as a constituent of a mixture from which the (meth)acrylic acid has to be removed (cf., for example, DE-A 10316465). However, as a consequence of the high affinity of (meth)acrylic acid to water, a separation of (meth)acrylic acid from steam present in the product gas mixture is particularly energy-intensive. It would therefore be advantageous from this aspect for the steam fraction in the charging gas mixture to be very low.
A further point is that the reaction gas mixture, as it passes through the catalyst bed, has to overcome its resistance. The energy required for this purpose has to be supplied to the charging gas mixture beforehand (for example in the form of compressive work; however, the reaction gas mixture may in principle also be sucked through the catalyst bed).
It will be appreciated that the greater the amount of the charging gas mixture, the greater the total amount of work to be performed (especially when conducting cycle gas). From this aspect, not only would a small steam content be advantageous, but also a very low ratio of molecular oxygen present in the charging gas mixture to the saturated hydrocarbon precursor compound present in the charging gas mixture. This is all the more true when the oxygen source used is not pure oxygen or nitrogen-depleted air, but rather air itself, since each oxygen molecule in this case is additionally accompanied by four nitrogen molecules.
It would therefore be advantageous for a process for heterogeneously catalyzed gas phase partial oxidation of at least one saturated hydrocarbon precursor compound to (meth)acrylic acid to display its full performance even at comparatively low steam contents of the charging gas mixture and comparatively small molar ratios of molecular oxygen present in the charging gas mixture to the at least one saturated hydrocarbon precursor compound in the charging gas mixture.
It is now generally known that catalytically active multimetal oxides which contain the elements Mo and V, at least one of the elements Te and Sb, and at least one of the elements from the group consisting of Nb, Ta, W, Ce and Ti may occur in various crystalline phases (cf., for example, DE-A 10246119 and DE-A 10254279).
One of the possible crystalline phases, known as the k phase (with hexagonal structure) features an x-ray diffractogram which has high-intensity reflections at the 2Θ peak locations 22.1±0.5°, 28.2±0.5°, 36.2±0.5°, 45.2±0.5° and 50.0±0.3°.
A second specific crystal structure (orthorhombic structure) in which the relevant multimetal oxide active compositions may occur is generally referred to as the i phase. Its x-ray diffractogram features, inter alia, high-intensity reflections at the 2Θ peak locations 22.2±0.5°, 27.3±0.5° and 28.2±0.5°, but, in contrast to the k phase, does not exhibit a high-intensity reflection at the 2Θ peak location 50.0±0.3° (cf. DE-A10119933 and DE-A 10118814).
The customary preparative processes of the relevant multimetal oxide compositions (for example the preparative processes of EP-A 1192987, EP-A 529853 and EP-A 603836) normally provide neither pure k phase nor pure i phase, but rather mixed crystal structures which are an intertwined mixture of k and i phase in which the k phase fraction normally dominates.
A measure of the i phase fraction in these mixed crystal structures is the intensity ratioR=Pi/(P+Pk)where Pi is the intensity of the reflection i at 2Θ=27.3±0.5° and Pk is the intensity of the reflection k at 2Θ=28.2±0.5° in the corresponding x-ray diffractogram.
Particularly high i phase fractions are present when 0.55 or 0.65≦R≦0.85. Pure i phase is present when the 2Θ peak location, 50.0±0.3°, additionally exhibits no reflection (cf., for example, DE-A 10246119).
Increased i phase fractions can be generated, inter alia, by washing the mixed crystal structures containing k and i phase with suitable liquids, for example aqueous nitric acid.